Frequency modulation receiver



p 1941. M. G. CROSBY 2,256,077

v FREQUENCY MODULATION RECEIVER Filed NOV. 17, 1939 INVENTOR MURRAY 6.0 sax BY M ATTORNEYv Patented Sept. 16, 1941 FREQUENCY MODULATION RECEIVER Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 1'7, 1939, Serial No. 304,888 5 Claims. (01. 250--20) This application concerns a new and improved frequency or phase modulation limiter, discriminator and detecting system. This application includes new and improved means for changing the detector circuits so that they may be used to demodulate amplitude modulated waves.

The single figure of the attached drawing illustrates my combined frequency or amplitude modulated wave receiver.

Wave length or amplitude modulated waves are fed to the terminals Ill. The supplied wave energy may be derived from the intermediatefrequency amplifier of a superheterodyne receiver which, if desired, includes AFC and AVC circuits. The first amplifier tube I6'is coupled at its input to an impedance I2 across the terminals l0 and at its output to the grid 20 of a cathode-driven limiter tube 24, by means of a tuned impedance coupling 30 which is suitably damped by resistance 3| so as to be substantially flat over the frequency range of the received signal. The cathode-driven limiter is coupled by coupling condenser 32 to the tube It and by condenser 34 to an amplifier tube 36 and uses the principles described in my United States application #273,460, filed May 13, 1939, and my United States application #275,151, filed May 23, 1939. The output of the first limiter is fed through a potentiometer 38 t the second amplifier 36 which feeds the second limiter tube 39. The limiter 39 is'coupled to tube 36, and tube 36 has a tuned output circuit 40 similar to tuned circuit 36. The potentiometer 38 adjusts the level from the output of the first limiter 24 to the optimum level which produces the best limiting effect in the second limiter 39. In this way,

the maximum degree of overall limiting is obtained.

The output of the second limiter 39 is fed to the switch S which cuts the limiter in and out through a fixed potentiometer 44. That is, when the switch S is on contact 45 excitation is supplied directly to the grid 56 of tube 52, whereas with the switch S on contact 46, the limiters are inserted in the path between input terminals [0 and the grid 50 of tube 52. The switch S is placed on contact 45 when amplitude modulated waves are being received. With this fixed potentiometer 44 and the two potentiometers 38 and 53 which are respectively in the input of the first amplifier l6 and the input of the amplifier 52, the input to the limiter may be adjusted to optimum with the aid of diode currents in the following manner: First, the limiter is switched in, by moving switch S to point 46,

and the input 55 to the first amplifier is set, by adjusting potentiometer l2, to the point which gives the best limiting. The word limiter as used here designates the apparatus between input In and potentiometer 53 or the separate limiters 24 and 39, the particular meaning being apparent from the context. The potentiometer 38 at the input of the second amplifier 36 is also set at its optimum position. This optimum setting is obtained by applying an amplitude modulation signal to the input terminals l6 and measuring the reduction in percentage of amplitude modulation which is effected by the limiter. With the limiter so adjusted, a constant output will appear at the output of the final limiter tube and this constant output will be limited to a maximum degree when a certain voltage, which we will call E, is applied to the input potentiometer l2 feeding the first amplifier [6. It is the purpose of the fixed p0- tentiometer 44 to reduce this constant output to a value which is equal to the signal input voltage E, to the first amplifier input potentiometer l2. The input at ID to the limiter will then be at its proper level when the diode currents with the limiter in are the same as the diode currents with the limiter out. Such a procedure is necessary so that the optimum input level will be fed to the limiter.

p The individualcathode-driven limiters 24 and 39 which have been described more in detail in my above-identified application #275,151 operate to efiect limiting of the negative and positive half cycles of the wave energy impressed thereon, in this respect difiering from the prior art as far as I know.

The amplifier tube 52has in its output a discriminator circuit which, as appears more in detail hereinaftenconverts the phase or frequency modulations on the wave energy to amplitude modulations thereon. This discriminator is similar to circuits used heretofore in automatic frequency control practice and the principle of the circuit involved here is substantially as described in S. W. Seeleys United StatesPatent #2,121,103 granted June 21, 1938.

This discriminator circuit includes a transformer 54 having a tuned primary 56 coupled to the anode 58 of tube 52 and a tuned secondary 66 coupled to the grids 62 and 64 of a pair of infinite impedance detector tubes VI and V2. A point on the secondary winding is connected by a resistance 65 to the cathodes of tubes VI and V2 while the cathodes are connected together by output resistances 66 and 68. Output voltages which'represent the frequency modulations are V supplied from the high audio frequency ends resistances 66 and 68 (when frequency modulation is received) to filter circuits Fl and F2 and thence to the Hiand12. V V

The particular discriminator shown in the diagram is somewhat different than the. usual type. The usual type has a choke coil from the mid-tap of the secondary winding-.69 to ground, but here a resistor 65 is used. The use of. a regrids Hand 1'5 of amplifier; tubes sistor in this position is made possible by the fact 7 that infinite-impedance diodes VI and V2 are used. This type of diode has a high input impedance which does not draw current so that a resistance in the return circuit maybe used and yet no potential drop be produced therein. The primary winding 56 is tuned, to pass aband of frequencies and the bandpass characteristics are improved by the damping resistance 57.. 'The' secondary winding 68 is similarly tuned and dampedf I W r The wave energy is supplied by coupling condenser -59 'cophasallyto'the grids 62 and Hand also induced into winding 60' and supplied antiphasally to the grids 62 and 64. When the frequency changes due to modulation there is no change-in thephase of the voltages supplied by 59 to the detectors but there is a change in the phase of the induced voltages supplied by 69 to V the. detectors. This produces on the detectors resultant voltage-the amplitude of which varies as the frequency modulations on the wave energy'from the output of tube 52 vary.

' The infinite-impedance detectors have a novel connection in that a phase reversal of the decondenser H to ground. The output of the filter feeds the balancing potentiometer BP through a blocking condenser 13. This balancing potentiometer is used for balancing thedetected outputs from. the two differential detectors. A second twin-triode l0, 12 is used as an audio amplifier with its plate circuits connected in push-pull by the transformer l8. The

:cathode biasingresistors 1.9 are unbypassed so series resistor 85and.8'l,and shunt capacitors 86 and 88 feed the detected energy from the cathode and plate circuits of the tube to the two points of the switch marked F-M and A-M (indicating frequency modulation and famplitudev modulation, respe'ctively) The arm 1 of the switchSl feeds the grid 16 of the audio amplifier 12 through blocking condenser 96. I

Amplitude modulation may be received on this type .of back-to-back frequency modulation rete cted output is obtained by switching one of the push -pull amplifier tubes, say 12, from the V v cathode output resistor '68 to the plate 61 of the same detector V2. This allows the reception of either frequency of amplitude modula- 'tion by merely throwing switches S and SI.

Thus, amplitude modulation 'is received by switching the limiter out by means of Sand throwing theamplifier input switch SI to-the contact point A-'-''M. This produces a phase reversal of the output'of tube V2 and connects the 'push pull amplifier so' that it is "fed theamplitude modulation signal in the proper pushpull: relationship'to produce the desired output.

The phase reversal required forthis proper-relationship is obtained by taking the output of 7 one detector, 'sayVl, from its cathode circuit and the output offthe other detector, say V2.

from its plate circuit. The primary 56 of the discriminator transformer 54 is tuned with a fixed condenser 55 and'a trimmer condenser 55.

Damping of theprimary is accomplished partially by the resistor 51 across it and partially by the resistor which is connected betweenthe high side of the primary and ground through; the blocking condenser 59. The variable con-' denser G ltunes the secondary 60 which, has the sections on either side of the mid-tap damped with resistors 63. Tubes VI and-VZare thetwo triodes of -atwin-triode,=which are used as infinite-impedance diodes for'the differential detectors. Cathode resistors 66 and 68 have the meters Min the cathode circuits to aid i'ntuning since theircurrents indicate the in- The condensers B6 across the cathode resistors are the diodeby-pass condensers. The output of the upper' tune condition by a balance.

diode'isfeddirectly from the cathode through This filterconsists ofzthe resistor- 69 and the the discriminator.

ceiver if. the detected outputs areconnectedin parallel instead of in push-pull. Normally, w1th the push-pull conection of the audio ampl fiers,

connecting the amplifier inputs to the cathodes only would be theadjustment for frequency modulation; reception- However, by utilizing.

the property of phase reversal effected between the cathode and plate circuits of a vacuum tube, either polarity of combination may bechosen by. 'switchingfthe input of one amplifier from the cathode to the plate circuit of one of the infinite-impedance diodes. When frequency modulation is being received;

the limiter is switched inby closing S on'contact 56' and the amplifier switch SI is'placed in the F-M position. This connects the detected outputs in push-pull so that they will be properly added. The push-pull relationship is required for frequency modulation reception because of the back-to-back action produced by v In this action voltages passed by sloping filters of opposite slope are fed to the detectors so that the resulting amplitudje" modulations will be degrees out of phase and will oppose and cancel.

The potentiometer BP at the input of one of the push-pull amplifiers hasthe function of ad'- justing the relative outputs from the two detectors Vi and V2 so that the back-to-back balance will be obtained. The resistors 69 and; 85, together with the condensers H and 86 form resistance filters whichremove the intermediatefrequency from thegrids of the audio-ampli; fiers.rr 7 7 An important advantage of this type of phase reversal which "is accomplished by switching;

from the cathode to the plate circuit. of one of the'detector tubes is the fact that the amplifiers. following the detectors may be connected in push pull. This tends to reduce harmonic distortion encountered in the amplifiers and also:

allows a better fidelity for a given amount of iron and copper in the output transformer due to the fact that the magnetizing effect of the permanent plate current is balanced out.

What is claimed is:

1. In a receiver, an input impedance, means for impressing modulated wave energy thereon, a transformer comprising a tuned primary coupled to a tuned secondary, means coupling said tuned primary to said impedance, a pair of electron discharge tube systems each having an anode, a cathode, and a control grid electrode, means connecting the secondary winding of said transformer between said control grids, a resistance connecting a point on said secondary winding to the cathodes of said tube systems,

means coupling the control grids of said tube systems to said impedance, impedances connected between the cathodes of said tube systems, an output impedance connected with the anode of one of said tube systems, a utilization circuit, and switching means for connecting said utilization circuit in shunt to the impedances between the cathodes of said tube systems or in shunt to one of said impedances and to the output impedance connected with the anode of the other of said tube systems.

2. In a receiver which may be used for amplifying and demodulating frequency modulated wave energy or amplitude modulated wave energy, an input impedance, an electron discharge device having input and output elec trodes, a limiter, switching means for connecting the input electrodes of said electron discharge device directly to said input impedance or by way of said limiter to said input impedance, a pair of electron discharge tube rectifiers each having a control grid, an anode and a cathode, an output impedance connected with the output electrodes of said first named tube device, means resonant to the mean frequency of the wave energy being received for impressing voltages from said output impedance in phase displaced relation on said control grids, means independent of variations in the frequency of the wave energy being received for impressing voltages from said output impedance in phase on said control grids, output impedances connected between the cathodes of said rectifier tubes, a utilization circuit, a pair of amplifying tubes having output electrodes connected in push-pull relation, said amplifier tubes having input electrodes, filter circuits coupling the impedances between the cathodes of said rectifier tubes in push-pull relation to the input electrodes of said amplifier tubes, said filter circuits serving to remove from the output of said detectors voltages of a frequency greater than the modulation potentials on the wave energy being received, an output impedance coupled with the anode of one of said rectifier tubes, and switching means for interrupting the connection between the input electrodes of one of said amplifying tubes and one of said impedances between the cathodes of said tubes and connecting the input electrodes of said one of said amplifier tubes to the anode of one of said rectifier tubes when the direct connection is established between the input electrodes of said discharge device and said input impedance.

3. In a frequency modulated or amplitude modulated wave receiver, a pair of electron discharge rectifier tubes having input electrodes including a cathode and output electrodes including an anode and said cathode, two output impedances connected between the cathodes of said devices, an output impedance connected between the anode and cathode of one of said devices, a frequency discriminating circuit 0011-, pled to said input electrodes, wave amplitude limiting means, means for impressing modulated wave energy onvsaid frequency discriminator circuit, means for including said wave amplitude limiting means with said last named means when the impressed wave energy is modulated in frequency, a pair of amplifying devices having output electrodes connected in an output circuit, said amplifying devices having input electrodes, means for coupling the input electrodes of said devices to said two impedances connected between the cathodes of said rectifier tubes, and means for interrupting the coupling between the input electrodes of one of said devices and one of said two impedances and coupling the input electrodes of said one of said devices to said impedance between the anode and cathode of said rectifier device when said wave amplitude limiting means is excluded from said second named means.

4. In a radio receiver adapted to receive frequency modulated carrier waves, a detection network comprising at least two electron discharge devices, each device including at least a cathode, a wave input electrode and an output electrode, a resistive impedance path connected between the cathodes of said devices, an intermediate point of said path being at a relatively fixed potential whereby the path is divided into two sections, each of said sections being unbypassed, a discriminator input circuit, means connecting each of said input electrodes to said input circuit, means applying modulated carrier waves to said input circuit, means establishing each of said input electrodes at said fixed potential whereby wave voltage developed across each of said sections is degeneratively applied to its respective input electrode, a direct current energizing circuit connected to each of the output electrodes of said devices, a modulation voltage utilization network comprising a pair of electron discharge devices, each device of said pair including at least a cathode, control grid and anode, a modulation voltage connection from one control grid to the cathode end of one of said two sections, a second modulation voltage connection from the second control grid to the cathode end of the second of said two sections, means connecting the anodes of said pair of devices in push-pull relation.

5. In a radio receiver adapted to receive fre quency modulated carrier waves, a detection network comprising at least two electron discharge devices, each device including at least a cathode, a wave input electrode and an output electrode, a resistive impedance path connected between the cathodes of said devices, an intermediate point of said path being at a relatively fixed potential whereby the path is divided into two sections, each of said sections being unby-passed, a discriminator input circuit, means connecting each of said input electrodes to said input circuit, means applying modulated carrier waves to said input circuit, means establishing each of said input electrodes at said fixed potential whereby wave voltage developed across each of said sections is degeneratively applied to its respective input electrode, a direct current energizing circuit connected to each of the output electrodes of said devices, a modulation voltage utilization network comprising a pair of electron discharge devices, each device of said pair inone control grid to the cathode end of oneof said two sections, asecond modulation Voltage connection from the second control grid to-thecathode end of the second of said two sections, means connecting the anodes of said pair of devices in push-pull 'relation, and-means for selectively disconnecting oneof said two modu- 1ation-vo1tage connections from its respective path section and connecting it instead to the output electrode of its respective one ofsaid detector devices for reception of amplitude modulated carrier waves.

MURRAY G. CROSBY. V 

